1. Field of the Invention
The present invention relates to an ink-jet print head which discharges liquid through an orifice to form liquid droplets. More specifically, the present invention relates to an ink-jet print head which applies thermal energy to liquid to cause a state change in liquid to discharge liquid. The present invention also relates to a method of producing such a print head, and to a printing apparatus with this type of print head, such as a copy machine, facsimile machine, printer and textile printing device.
2. Related Background Art
Of various types of ink-jet print heads, the ink-jet printing method disclosed in Japanese Laid-Open Patent Application No. 54-51837 has a unique feature different from the other types of ink-jet printing methods in that liquid droplets are discharged upon application of the thermal energy to liquid. In the method disclosed in the application, the liquid is heated by means of thermal energy so as to generate a bubble, which creates the force to form the liquid droplet through an orifice at the leading end of the print head. Then the droplet is deposited on a recording medium to print information thereon.
In general, a print head applied to the above-mentioned method comprises an orifice through which liquid is discharged, a liquid discharge portion communicating with the orifice, having a liquid flow path including as a part a heat acting section for applying the thermal energy to the liquid to discharge the liquid droplets, a heat generating resistor as an electrothermal energy conversion element which is thermal energy generating means and an electrode, and an upper protecting layer for protecting the heat generating resistor and the electrode from ink.
The heat generating resistor, the electrode, and the upper protecting layer are generally formed by sequentially depositing as thin films on a substrate. The thin films can be formed by a technique such as sputtering or CVD (chemical vapor deposition).
If the heat generating resistor layer is in direct contact with the recording or printing liquid, electrical current may flow through the printing liquid in accordance with a value of the electric resistance of the printing liquid and the printing liquid may be electrolyzed due to electrical current through the printing liquid. In addition, the printing liquid reacts with the heat generating resistor layer upon energization of the heat generating resistor layer so that the heat generating resistor layer may be damaged or the resistance thereof may vary.
Conventionally, to solve the above problems and to improve the reliability and the durability for long-time repeated use a protection layer made of a high acid-resistance material such as SiO2 on the heat generating resistor layer is provided so as to prevent the heat generating resistor layer from being in direct contact with the printing liquid.
To achieve the requirements such as prevention of the damage of the heating resistor layer and prevention of short-circuiting between the electrodes, the protection layer covering the thermal energy generating means should have no defects such as pin holes in the film so that the heating resistor layer and the major portions of the electrodes can be covered uniformly with the protection layer.
However, because the electrodes are formed on the heating resistor layer as described above, there are steps between the electrodes and the heating resistor layer. In conventional techniques of forming a thin film, it is difficult to form a uniform protection layer on the electrodes and on the heating resistor layer having such steps. The thickness of the protection film tends to be thinner at the steps as shown in FIG. 3, which may sometimes cause exposure of a portion of the electrodes or the heating resistor layer.
In such a state where the step coverage is not good enough, the exposed portion of the heating resistor layer may be in direct contact with the printing liquid. As a result, electrolysis of the printing liquid may occur, and reaction between the printing liquid and the heating resistor layer may occur, which may result in the damage of the heating resistor layer. Moreover, due to the fact that the thickness of the protection layer tends to be non-uniform near the step regions, thermal cycling induces the localized thermal stress in a part of the protection layer, which results in cracks in the protection layer. If such cracking occurs, the printing liquid may penetrate through the cracks to reach the heating resistor layer and thus the heating resistor layer may be damaged.
A common known way to solve such problems is to thicken the protection film so as to improve the step coverage and to reduce the pin holes. However, the thickening of the protection film causes another problem that the thermal resistance between the heating resistor layer and the bubbling surface increases, which results in low thermal response of the bubbling surface. Moreover, it becomes necessary to apply higher electrical power to the heating resistor layer, which causes low durability of the heating resistor layer. Thus, there has been the need to develop a technique which can form a protection layer having good step coverage without increasing the thickness of the protection layer.
There have been various attempts to achieve this requirement. For example, Japanese Laid-Open Patent Application No. 60-234850 discloses a bias sputtering technique which can form a protection film having good step coverage. Japanese Laid-Open Patent Application Nos. 62-45283, and 62-45237 disclose a technique in which the step coverage is improved by altering the shape of the steps by means of etching back or sputter-etching the protection film which has been already deposited. In the technology disclosed in Japanese Laid-Open Patent Application No. 62-45286, the protection film is re-flowed to improve the step coverage. In HP Journal, May, 1985, there is disclosed a technique in which electrodes are formed in a tapered shape to improve the step coverage.
However, each of these techniques has its own problems. For example, in bias sputtering, it is difficult to control the thickness of a film, and thus good reproducibility cannot be obtained. Another problem of this technique is that contamination or dust occurs around a target material. Etching back and sputter-etching techniques result in an increase in the number of processing steps, which further results in a decrease in throughput or production yields. On the other hand, re-flowing requires a high temperature which degrades the reliability of aluminum electrodes. In tapered-shape electrodes, it is difficult to obtain good uniformity and reproducibility in the tapered shape, which causes the variations in resistance value.
As described above, there are no conventional techniques which can form a high quality protection film applicable to an ink-jet head with a high production yield.
In view of the above, it is a major object of the present invention to provide a method of producing an ink-jet print head having good durability which can be produced by using common production processes.
It is another object of the present invention to provide a method of producing an ink-jet print head having good step coverage which can perform good thermal response.
It is still another object of the present invention to provide an ink-jet print head produced by the above production methods.
It is a further object of the present invention to provide a printing apparatus using the above ink-jet print head.